Liquid crystal display with optical compensator

ABSTRACT

A multi-colored pixel for a twisted nematic liquid crystal display including red, green, and blue subpixels, wherein each subpixel includes a pair of substrates, a pair of polarizers, opposing electrodes, and a color personalized retardation film which compensates for the different wavelength of each color. The personalized retardation films of the different color subpixels results in elimination of the multi-gap approach and substantially eliminates the problem of different color leakages at different viewing angles, including normal. Also, one polymer based element, preferably a polyimide, functions as both a color filter and a retardation film in certain embodiments of this invention.

This application is a division of Ser. No. 08/595,068 filed Feb. 1, 1996and a division of Ser. No. 08/451,962 filed May 26, 1995 now U.S. Pat.No. 5,818,615 and a division of Ser. No. 08/160,731 filed Dec. 2, 1993now U.S. Pat. No. 5,499,126.

This invention relates to the design of a liquid crystal display havingat least one retardation film therein. More particularly, this inventionrelates to the design of a polychromatic or multicolored liquid crystaldisplay and techniques for eliminating color leakages and maximizing thefield of view of such displays.

BACKGROUND OF THE INVENTION

Liquid crystal materials are useful for electronic displays becauselight traveling through a layer of liquid crystal (LC) material isaffected by the anisotropic or birefringent value (ΔN) of the material,which in turn can be controlled by the application of a voltage acrossthe liquid crystal material. Liquid crystal displays are desirablebecause the transmission or reflection of light from an external source,including ambient light and backlighting schemes, can be controlled withmuch less power than was required for the illuminance materials used inother previous displays. Liquid crystal displays are now commonly usedin such applications as digital watches, calculators, portablecomputers, avionic cockpit displays, and many other types of electronicdevices which utilize the liquid crystal display advantages of long lifeand operation with low voltage and power consumption.

The information in many liquid crystal displays is presented in the formof a matrix array of rows and columns of numerals or characters, whichare generated by a number of segmented electrodes arranged in such amatrix pattern. The segments are connected by individual leads todriving electronics, which apply a voltage to the appropriatecombination of segments to thereby display the desired data andinformation by controlling the light transmitted through the liquidcrystal material. Graphic information in, for example, avionic cockpitapplications or television displays may be achieved by a matrix ofpixels which are connected by an X-Y sequential addressing schemebetween two sets of perpendicular conductor lines (i.e. row and columnlines). More advanced addressing schemes use arrays of thin filmtransistors or diodes which act as switches to control the drive voltageat the individual pixels. These schemes are applied predominantly totwisted nematic liquid crystal displays, but are also finding use inhigh performance versions of super twisted liquid crystal displays.

Contrast is one of the most important attributes determining the qualityof both normally white (NW) and normally black (NB) liquid crystaldisplays. In normally black (NB) LCDs, the primary factor limiting thecontrast achievable in these liquid crystal displays is the amount oflight which leaks through the display in the darkened or OFF state. Inthe NW mode, the primary factor limiting the contrast is the amount oflight which leaks through the display in the darkened ON state. Thisproblem is compounded in a bright environment, such as sunlight, wherethere is a considerable amount of reflected and scattered ambient light.In color liquid crystal displays, light leakage causes severe colorshifts for both saturated and gray scale colors. These limitations areparticularly important for avionics applications, where copilot viewingof the pilot's displays is important.

In addition, the legibility of the image generated by both normallyblack (NB) and normally white (NW) liquid crystal devices depends on theviewing angle, especially in a matrix addressed device with a largenumber of scanning electrodes. Absent a retardation film, the contrastratio in a typical NB or NW liquid crystal display is usually at amaximum only within a narrow viewing (or observing) angle centered aboutnormal incidence (0° horizontal viewing angle and 0° vertical viewingangle) and drops off as the angle of view is increased.

It would be a significant improvement in the art to provide a liquidcrystal display capable of presenting a high quality, high contrastimage over a wide field of view.

Several types of liquid crystal pixels or cells are in widespread use inflat panel displays. Active matrix addressing allows such displays topresent a full color image with high resolution. When viewed directly ata normal or on-axis viewing angle N (0° vertical viewing angle and 0°horizontal viewing angle), a liquid crystal display provides a generallyhigh quality output especially when the cell gap “d” is matched to thefirst transmission minimum, but the image degrades and exhibits poorcontrast at increased viewing angles. This occurs because liquid crystalcells operate by virtue of the anisotropic or birefringent effectexhibited by a liquid crystal layer which includes a large number ofanisotropic liquid crystal molecules. Such a material will be positivelyuniaxially birefringent (i.e., the extraordinary refractive index islarger than the ordinary refractive index) with an extraordinaryrefractive index associated with the alignment of the long molecularaxes. The phase retardation effect such a liquid crystal material has onlight passing through it inherently varies or increases with theinclination angle of light, leading to a lower quality image at largerviewing angles (see, e.g. Penz, Viewing Characteristics of the TwistedNematic Display, Proceeding of the S.I.D., Vol. 19, p. 43 (1978);Grinberg, et al., Transmission Characteristics of a Twisted NematicLiquid Crystal Layer, Journal of the Optical Society of America, Vol.66, p. 1003 (1976)). By introducing an optical compensating element (orretarder) into the liquid crystal pixel or cell, however, it is possibleto correct for the unwanted angular effects and thereby maintain highercontrast at both normal and larger viewing angles than otherwisepossible.

The type and orientation of optical compensation or retardation requireddepends upon the type of display, normally black or normally white,which is used.

In a normally black (NB) twisted nematic display, the twisted nematicliquid crystal material is placed between polarizers whose transmissionaxes are parallel to one another. Such NB displays may be either frontX-buffed or rear X-buffed. The first and second LC buffing zones arepreferably perpendicular to one another thereby necessitating one of thebuffs being perpendicular relative to the polarizer axes. If the firstbuff zone is perpendicular to the first polarizer transmission axis thenthe display is rear “X-buffed.” Otherwise, it is front “X-buffed.”

In the unenergized or OFF state (no voltage below the threshold voltageV_(th) is applied across the liquid crystal material), normally incidentlight from the backlight is first polarized by the first polarizer andin passing through the pixel or cell has its polarization directionrotated by the twist angle of the liquid crystal material dictated bythe buffing zones. This effect is known as the waveguiding or twistingeffect. The twist angle is set, for example, to be about 90° so that thelight is blocked or absorbed by the second or output polarizer. When avoltage is applied via electrodes across the normally black pixel, theliquid crystal molecules are forced to more nearly align with theelectric field, eliminating the twisted nematic symmetry of the LCmaterial. In this orientation, the optical molecular axes of the liquidcrystal layer molecules are perpendicular to the cell walls. The liquidcrystal layer then appears isotropic to normally incident light,eliminating the waveguiding effect such that the light polarizationstate is unchanged by propagation through the liquid crystal layer sothat light can pass through the second or output polarizer. Patterns canbe written in the NB display by selectively applying a variable voltageto the portions of the display which are to appear illuminated.

When viewed in the OFF state at both normal N and other viewing angles,however, the dark unenergized areas of a normally black display willappear colored because of angle dependent retardation effects for lightpassing through the liquid crystal layer at such angles. Contrast can berestored by using a compensating or retarding element which has anoptical symmetry similar to that of the twisted liquid crystal layer butwhich reverses its effect. One method is to follow the active liquidcrystal layer with another twist liquid crystal cell of reversehelicity. Another, in an NB cell, is to use one or more plate retardercompensators each having a constant birefringent value throughout thepixel. These compensation methods work because the compensation orretardation element shares an optical symmetry with the twisted nematicliquid crystal material in that both are preferably uniaxialbirefringent materials having extraordinary axes orthongonal to thenormal light propagation direction. These approaches to compensationhave been widely utilized because of the ready availability of materialswith the required optical symmetry. Reverse twist cells employ liquidcrystals while retardation plates are readily manufactured by thestretching of the polymers such as polyvinyl alcohol (PVA). Regardingthe reverse twist cell compensation technique discussed above, thisrequires the insertion of a second liquid crystal cell into the opticalpath, adding significant cost, weight and bulk to the display.

Despite the effectiveness of these compensation techniques, there aredrawbacks to these approaches associated with the normally blackoperational mode. The appearance of a normally black display is verysensitive to cell gap “d.” Consequently, in order to maintain a uniformdark appearance in the OFF state, it is necessary to match the thickness“d” of the liquid crystal layer to the first transmission minimum ofeach particular wavelength or color used in the pixel. This isillustrated in prior art FIG. 1 (see, for example, U.S. Pat. No.4,632,514) which shows a multi-colored pixel for a liquid crystaldisplay including a blue subpixel, a green subpixel and a red subpixel,wherein the thickness or cell gap “d” of the liquid crystal layer 15varies according to the color or wavelength of each subpixel so as tomatch “d” to the first transmission minimum of each color. Suchmulti-gap displays are very difficult and expensive to manufacture.

Therefore, it would be highly desireable to provide a liquid crystaldisplay including red, green, and blue subpixels as shown in FIG. 1,which has good color contrast ratios and compensates for the differentcolor wavelengths but does not require varying the thickness “d” of theliquid crystal layer according to each color so as to selectively match“d” to the first transmission minimum of the wavelength of each subpixelcolor (red, green, blue).

Turning now to NW LCD cells, in a normally white liquid crystal displayconfiguration, a twisted nematic cell preferably having a twist angle ofabout 90° is placed between polarizers which have crossed orperpendicular transmission axes, such that the transmission axis of eachpolarizer is parallel or perpendicular to the buffing direction oforientation of the liquid crystal molecules in the interface region ofthe liquid crystal material adjacent to each polarizer. In other words,NW cells can be either P-buffed wherein both polarizer axes are parallelto their respective adjacent buffing zones, or X-buffed wherein bothpolarizer axes are perpendicular to their respective buffing zones. Thisorientation of the polarizers reverses the sense of light and dark fromthat of the normally black display discussed above. The OFF orunenergized (no applied voltage above V_(th) across the liquid crystalmaterial) areas appear light in a normally white display, while thosewhich are energized appeared dark. The problem of ostensibly dark areasappearing light or colored when viewed at large angles still occurs,however, the reason for it is different. Either positive or negativebirefringent retarders may be used to correct the NW displays, dependingupon their orientation. In the NW energized darkened areas, the liquidcrystal molecules tend to align with the applied electric field. If thisalignment were perfect, all of the liquid crystal molecules in the cellwould have their long axes normal to the glass substrate or cell wall.In the energized state, the normally white display appears isotropic tonormally incident light, which is blocked by the crossed polarizers,thus, resulting in a darkened pixel or subpixel.

The loss of contrast with increased viewing angles in NW pixels ordisplays occurs primarily because the homeotropic liquid crystal layerdoes not appear isotropic to off axis or off normal light. Lightdirected at off normal angles propagates in two modes due to theanisotropy or birefringence (ΔN) of the liquid crystal layer, with aphase delay between these modes which increases with the incident angleof light. This phase dependence on the incident angle introduces anellipticity to the polarization state which is then incompletelyextinguished by the second polarizer, giving rise to light leakage.Because of the NW symmetry the birefringence has no azimuthaldependence.

Accordingly, what is often needed in normally white liquid crystal cellsis an optical compensating or retarding element which would introduce aphase delay opposite in sign to that caused by the liquid crystal layer,thereby restoring the original polarization state, allowing the light tobe blocked by the output polarizer. Optical compensating elements orretarders with such NW symmetry and often negative birefringence areknown in the art and are disclosed, for example, in U.S. Pat. Nos.5,196,953, 5,138,474, and 5,071,997, the disclosures of which are herebyincorporated herein by reference. It is known that the polyimides andco-polyimides disclosed by aforesaid U.S. Pat. No. 5,071,997 can be usedas retarding elements in NW liquid crystal displays and are said to becustom tailorable to the desired negative birefringent values withoutthe use of stretching.

Turning once again to FIG. 1, there is illustrated a prior art normallyblack liquid crystal display pixel including three colored subpixels.Optical radiation from a radiation source is applied to the liquidcrystal display pixel of FIG. 1. The applied optical radiationschematically illustrated as 2A, 2B, and 2C, is typically from a singlesource, but is shown in FIG. 1 in terms of the component or subpixelunits of the display pixel. The optical radiation first passes throughfirst linear polarizer 14. The optical radiation is then applied to theliquid crystal cell 10. The liquid crystal cell 10 is bounded by twotransparent glass substrates 11 and 12. On the interior surface of theglass substrate 12 are transparent conducting regions 18A, 18B, and 18C.These conducting regions are electrodes for applying an electric fieldto the liquid crystal layer 15 of each subpixel color component unit ofthe display pixel. The blue subpixel has a blue optical filter 16A; thegreen subpixel has a green optical filter 16B; and the red subpixel hasa red optical filter 16C. These optical filters are coupled to thesecond glass substrate 11.

Deposited on the optical filters is a transparent conducting material 17which acts as the second electrode for each subpixel of the liquidcrystal pixel. A power supply 4 is provided to illustrate that apotential can be applied to the liquid crystal material 15 whichoccupies the region between the electrodes 18A, 18B, and 18C and thesecond electrode 17. As will be clear to those familiar with liquidcrystal displays, the power supply 4 is typically replaced by addressingcircuitry for applying a predetermined voltage to each of the subpixelelectrodes. In this manner, an image can be displayed to a viewer (orobserver).

The optical radiation 19A, having been linearly polarized by the firstpolarizer 14, is rotated about 90° during transmission through theliquid crystal material 15 between the first electrode 18A and secondelectrode 17. The linearly polarized optical radiation 19B and 19C aresimilarly rotated about 90° in the different color subpixels of thepixel. The optical radiation, after transmission through the liquidcrystal material 15 passes through one of the color filters 16A, 16B,and 16C. The optical color filters select the color components for theirrespective subpixels to be transmitted by the color subpixels of theliquid crystal display. However, the different wavelengths (e.g. red,green, and blue) are affected to different extents by the birefringenceof the LC material thereby necessitating the multi-gap configurationshown in FIG. 1 and creating different relative color leakages atdifferent viewing angles.

After transmission through the liquid crystal material, the opticalradiation is transmitted through the retardation plates 21 and 22. Theoff axis transmission, as discussed above, becomes increasinglyelliptically polarized with increased angle, a result of thebirefringence of the liquid crystal material. The result of thiselliptical polarization is a reduction of radiation contrast as afunction of angle about the normal axis N after transmission of theradiation through the second linear polarizing plate 13. In order tocompensate for the angular dependent reduction in contrast, retardationplates 21 and 22 of constant retardation value are interposed betweenthe substrate 11 and the polarizer 13 as shown in FIG. 1. The presenceof the retardation plates 21 and 22 results in a decrease in theelliptical polarization of the radiation applied to the linearpolarizing plate 13. Consequently, the angle dependent variation incontrast ratio of the radiation transmitted through the second linearpolarizing plate 13 is improved.

Furthermore, as shown in FIG. 1, the multi-gap aspect of this prior artpixel requires the thickness “d” of each subpixel being selected so asto match the optical path difference (d·ΔN)÷λ of the liquid crystal cell15 to the first transmission minimum of each respective color of thethree subpixels. Accordingly, because each color (red, green, and blue)has a different wavelength and the birefringent value ΔN of the liquidcrystal material remains constant, the thickness “d” of each subpixelmust be adjusted accordingly so as to compensate for the differentwavelengths of each color and the cell is thereby optimized for thenormal viewing angle N. The normal viewing angle herein is shown byreference element “N” and means about a 0° horizontal and verticalviewing angle.

Reference next is to prior art FIG. 2, which illustrates schematicallyhow the light travels in the LCD of FIG. 1. As illustrated, the incomingradiation 2 is first transmitted through first linear polarizer 14. Thenext optically oriented region through which the optical radiationpasses is the first orientation film or surface 18S of the conductingplates with which the liquid crystal material 15 is in contact. Thesurface 18S has an orientation or buffing parallel to the first linearpolarizer 14. Ignoring for purposes of this discussion the controllableorientation of the actual liquid crystal material, the next opticallyoriented region through which the optical radiation is transmitted isthe second orientation film or buffed surface 17S of the secondconducting electrode 17, the second surface to which the liquid crystalmaterial 15 is exposed. The surface 17S is oriented or buffed in adirection perpendicular to the surface 18S to which the liquid crystalis exposed thereby creating about a 90° twist in the LC material. Theretardation plate 21, having a constant anisotropic or birefringentvalue (ΔN) as well as a constant retardation value throughout all threesubpixels, has an optical axis oriented parallel to the orientation ofthe surface 17S, while retardation plate 22, also having a constantbirefringent value throughout all three subpixels, has its optical axisoriented at right angles to the axis orientation of retardation plate21. The retardation value of a retardation plate or film is determinedby the formula “d·ΔN,” wherein “d” is the thickness of the plate or filmand “ΔN” is the birefringent value of the plate. Finally, when the pixelis in the ON or energized state, the optical radiation is transmittedthrough the second linear polarizer 13 which is oriented parallel toretardation plate 22 and linear polarizer 14.

Referring next to FIG. 3, which is a graph illustrating the differenttransmission minimums of red, green, and blue wavelengths in a normallyblack liquid crystal cell, the percent optical transmission through aliquid crystal cell in the absence of an applied electric field as afunction of distance “d” in the liquid crystal material through whichthe optical radiation travels is illustrated for the typical colorcomponents. For substantially no transmission of optical radiation inthe OFF state, the transmission minimum for blue radiation occurs atapproximately a thickness of the liquid crystal material “d (blue)”, thetransmission minimum for green radiation occurs at a thickness of liquidcrystal material of “d (green)” which is greater than “d (blue)”, andthe transmission minimum for red radiation occurs at a thickness ofliquid crystal material of “d (red)” which is greater than “d (green)”.This difference in the minimum of the transmitted radiation of eachcolor is, as discussed above, the reason that the cell thickness “d” ofeach subpixel is different in the multi-gap configuration of FIG. 1.

A drawback of the prior art liquid crystal display discussed above suchas has been illustrated and discussed with reference to FIGS. 1-2, isthat the thicknesses “d” of the LC material must be finely adjusted tomatch the first transmission minimum of each color, and furthermore, theretardation film(s) 21 and 22 have a single retardation value applicableto all of the colored subpixels and do not take into consideration thedifferent wavelengths. Because of the constant retardation values of theretardation films for all of the subpixels, the result is that atdifferent viewing angles, there are different viewing leakages for thedifferent colors (red, green, and blue). The NB pixel shown in FIG. 1,for example, when viewed in the OFF state at a normal viewing angle Nexperiences a blue leakage, because the single constant value of theretardation plates or films is substantially matched to the greenwavelength at a normal viewing angle. However, at increased horizontalviewing angles, the pixel of FIG. 1 experiences green and red leakagewhile properly transmitting the blue color.

In the case of obliquely angled light traveling through the pixel shownin FIG. 1, the normal component or vector is twisted about 90° by theliquid crystal material but the horizontal component is twisted toanother angle dependent value. The purpose of the retardation plates 21and 22 shown in FIG. 1 is to correct the horizontal component which wasadversely affected by the liquid crystal material. However, theretarders shown in FIG. 1 have a single retardation value which does nottake into consideration the different wavelengths of each color (e.g.red, green, and blue) which have been affected differently by thebirefringence of the LC material. In other words, when using a retarderwith a constant retardation value, the overall viewing angle of themulti-gap pixel shown in FIG. 1 can be improved, but at differentviewing angles, the result is different viewing leakages for each color.

Prior art FIG. 4 illustrates a second type of known NB pixel whichincludes red, green, and blue subpixels. Normally incident light 101first passes through a first linear polarizer 103. First linearpolarizer 103 has a transmission axis parallel to the transmission axisof second linear polarizer 112, thereby defining a normally black (NB)liquid crystal display pixel. After being polarized by linear polarizer103, the light 101 then proceeds through a first transparent substrate104 and transparent subpixel electrodes 105. Each color subpixel has itsown electrode 105 which enables a selectively activated voltage to beapplied across each subpixel. After passing through electrodes 105, thenormally incident light 101 then proceeds into and through a liquidcrystal layer 109 having a thickness “d.” The liquid crystal layer 109,having a constant thickness throughout the entire pixel, has, at itsinterface with electrodes 105 a first orientation film (not shown)buffed in a direction substantially perpendicular to the transmissionaxis of the first polarizer 103. Opposing the first orientation film(not shown) is a second orientation film (not shown) disposed at theinterface of the liquid crystal material 109 and color filters 106-108.This second orientation film (not shown) is buffed in a directionsubstantially parallel to the transmission axes of both the first andsecond polarizers. The substantially crossed buffing directions of thefirst and second orientation films (not shown) creates about a 82°-100°twist in the liquid crystal layer 109. In other words, as normallyincident light passes through the liquid crystal material 109 from thefirst orientation film adjacent the electrodes 105 to the secondorientation film adjacent the color filters, the light is twisted about82°-100°. After proceeding through the liquid crystal layer 109, thelight then progresses through the aforesaid described second orientationfilm and the color filters 106-108 of the respective subpixels. The bluesubpixel includes a blue color filter 106, the red subpixel a red colorfilter 107, and the green subpixel a green color filter 108. Afterpassing through one of the color filters, the normally incident lightthen proceeds through a second transparent substrate 110, a retardationfilm 111, and a second or exit polarizer 112. The retardation film 111has a constant retardation value throughout the entire pixel. Afterpassing through the second polarizer 112 which has a transmission axisoriented parallel to the transmission axis of the first polarizer 103,the light proceeds toward a viewer who preferably views the resultinglight at an ON axis or normal viewing angle 113. The normal viewingangle N has its axis perpendicular to a plane defined by, for example,the polarizers 103 and 112 of the liquid crystal cell.

The cell gap or thickness “d” of this particular pixel is about 5.70micrometers and is matched to the first transmission minimum for thecolor green which has a wavelength of 550 nm. The retardation film 111has a constant birefringent value (ΔN) which is positive. The opticalaxis of the retardation film 111 is parallel to the buffing zone of thefirst orientation film and perpendicular to the transmission axes of thefirst and second polarizers 103 and 112. The principal drawback, as willbe described below, of this prior art pixel shown in FIG. 4 is that thedifferent wavelengths representative of the different colors are notcompensated for, the result being a variance in contrast between thecolors at different viewing angles.

FIGS. 5-7 are computer simulation graphs illustrating the effect of thepixel of FIG. 4, absent its retardation film, upon red, green, and bluewavelengths respectively.

FIG. 5 is a computer simulation graph illustrating the effect of thepixel of FIG. 4, absent its retardation film 111, on the red lightwavelength of 630 nm. The parameters used in simulating this effectshown in FIG. 5, include a cell gap “d” of 5.70 micrometers, a drivingON voltage of 4.0 volts, an OFF voltage of 0.9 volts, and the linearpolarizers 103 and 112 having transmission axes parallel to one anotherand perpendicular to the first buffing zone adjacent the electrodes 105.As can be seen in FIG. 5, the contrast ratio at normal (0° vertical, 0°horizontal viewing angle) is only about 30:1. Furthermore, as oneproceeds up and down the 0° horizontal axis (e.g. 0° horizontal, −40° to40° vertical), the contrast ratio never exceeds about 30:1 and quicklydrops below 30:1 at vertical viewing angles of about 7° and −15°. Thisgraph illustrates a twin peak effect meaning that while the contrastratio is poor at normal, it is improved horizontally on both sides ofnormal. In other words, the contrast ratio at 0° vertical and 30°horizontal is about 130; and the contrast ratio at 0° vertical and −30°horizontal is about 110:1. As is evident by this graph illustrated inFIG. 5, the red wavelength of 630 nm incident upon the pixel of FIG. 4absent its retarder, suffers greatly at substantially normal viewingangles, and all vertical viewing angles where the horizontal viewingangle is around 0°.

FIG. 6 is a computer simulation of the effect that the pixel of FIG. 4,absent its retardation film, has upon green light with a wavelength of550 nm. This simulation utilizes as parameters a cell gap of 5.70micrometers, an ON voltage of 4.0 volts, an OFF voltage of 0.9 volts,and parallel polarizer axes which are perpendicular to the first buffingzone adjacent the electrodes 105. Because the cell gap “d” of the FIG. 4prior art pixel is matched to the first transmission minimum of thegreen wavelength of 550 nm used in this simulation, the contrast ratioat normal (0° vertical, 0° horizontal) is very good at about 210:1. The30:1 contrast ratio curve extends along the 0° horizontal axis fromvertical angles of about −27° to about +30° thereby spanning a rangealong the 0° horizontal axis of about 57°. Furthermore, the 30:1contrast ratio curve extends along the 0° vertical axis from horizontalangles of about −37° to about +37°, thereby defining a horizontal rangealong the 0° vertical viewing axis of about 74°. The contrast ratiocurves of FIG. 6 for the color green are markedly superior to those ofFIG. 5 because the cell gap “d” of the FIG. 4 pixel is matched to thefirst transmission minimum for the color green, while being lower thanthe first transmission minimum of the color red. Likewise, because thecell gap of the FIG. 4 pixel is matched to the first transmissionminimum of the color green and is higher than that needed for the colorblue, the contrast ratio graph for the color blue described below withregard to FIG. 7 is inferior to that of the color green shown in FIG. 6.

FIG. 7 is a computer simulation of a graph illustrating the effect ofthe pixel shown in FIG. 4, absent its retardation film, on the bluewavelength at 480 nm. This graph uses parameters including a cell gap of5.70 micrometers, an ON voltage of 4.0 volts, an OFF voltage of 0.9volts, and polarizers having parallel transmission axes perpendicular tothe first buffing zone. As can be seen in FIG. 7, because the cell gap“d” of the FIG. 4 pixel is not matched to the first transmission minimumfor the color blue, the contrast ratio of the blue wavelength at normalis poor, being less than about 40:1. Furthermore, the 30:1 contrastratio curve extends along the 0° horizontal axis only to a limitation ofabout −8° vertical. Also, the same 30:1 contrast ratio curve extendsalong the vertical 0° axis to horizontal extents of only about −13° and+13°. As will be evident to those skilled in the liquid crystal displayart, this is a relatively poor contrast ratio curve indicative of theproblems of the prior art FIG. 4 pixel.

FIGS. 8-10 are computer simulation graphs illustrating the contrastratio curves of the prior art FIG. 4 pixel, including a retardation filmhaving a constant retardation value of 275 nm, with respect to thecolors red, green, and blue respectively. These three graphs utilizevoltage parameters including a 4.8 V on voltage, and a 0.2 V OFFvoltage. The use of a 275 nm retardation film within the FIG. 4 priorart pixel is not considered prior art, but is utilized in thesesimulation graphs for the purpose of later discussed comparison withcertain embodiments of this invention.

FIG. 8 illustrates the contrast ratios given the color red at awavelength of 630 nm by the prior art pixel shown in FIG. 4 including a275 nm retardation film. The contrast ratio at normal is only about30:1. The 30:1 contrast ratio curve extends along the 0° horizontalviewing axis to vertical viewing angles of about −35° and +12°. Again,the contrast ratio curves shown in FIG. 8 for the color red are verypoor because the pixel of FIG. 4 including its retardation film having aconstant retardation value of 275 nm, does not compensate for thedifferent wavelengths representative of the red, green, and blue colors.Accordingly, because the cell gap of the FIG. 4 pixel is matched to thefirst transmission minimum of the color green, thereby being below thefirst transmission minimum for the color red, the resulting contrastratios for the color red at normal and most other viewing angles arevery poor as illustrated in FIG. 8.

FIG. 9 is a computer simulation of contrast ratios for the color greenwavelength of 550 nm resulting from the pixel shown in FIG. 4 includinga retardation film having a retardation value of 275 nm. Because thecell gap of 5.70 micrometers is matched to the first transmissionminimum for the color green, the resultant contrast ratio curvesillustrated by FIG. 9 are relatively good. The contrast ratio at normalis about 270:1, while the 30:1 contrast ratio curve extends off thegraph along both the vertical and horizontal 0° viewing axes. Asdiscussed previously, the reason for the superior contrast ratios forthe color green in the FIG. 4 pixel, is that the cell gap in the pixelis matched to the first transmission minimum for the color green, andfurthermore, the retardation film has a retardation value of 275 nmwhich is also personalized to the color green.

FIG. 10 illustrates a computer simulation graph of contrast ratios forthe color blue wavelength of 480 nm propagating through the prior artpixel shown in FIG. 4. As can be seen in FIG. 10, because the cell gap“d” is not matched to the first transmission minimum of the bluewavelength, the contrast ratios are poor. At normal, for example, thecontrast ratio is only about 30:1. The 30:1 contrast ratio curve extendshorizontally along the 0° vertical viewing axis from about −26° to +26°.Furthermore, the 30:1 contrast ratio curve extends downward along the 0°horizontal axis only to about −9° vertical. Accordingly, it is clearthat the prior art pixel shown in FIG. 4 provides poor contrast ratiosboth horizontally and vertically for the blue wavelength.

It would clearly be a step forward in the art if a liquid crystaldisplay pixel could be provided which displayed good contrast ratios forall colors and eliminated the need for the multi-gap configuration shownin FIG. 1.

U.S. Pat. No. 5,179,457 discloses a liquid crystal display deviceincluding a phase plate disposed between a liquid crystal layer and alower electrode, wherein the phase plate has different amounts ofbirefringence at different positions thereby creating a color displaywithout using color filter(s). U.S. Pat. No. 5,179,457 does not discussusing such a phase plate in combination with color filters, and isdirected toward a different type of LCD than the present Invention.

U.S. Pat. No. 5,150,237 discloses an electrically controlledbirefringence (ECB) type LCD which utilizes a uniaxial medium having apositive anisotropy arranged between the liquid crystal layer and apolarizer, wherein the products of refractive index anisotropy andthickness of the uniaxial medium are different from each other inaccordance with the difference between displayed colors. The ECB displayof U.S. Pat. No. 5,150,237 is not directed toward a twisted nematic typeLCD which uses color filters as described by the instant invention.

U.S. Pat. No. 5,250,214 discloses a combination of a phase plate and anoptical color filter film wherein the phase plate includes a film ofliquid crystal polymer composition having polyester as a mainconstituent.

U.S. Pat. No. 5,229,039 discloses a polyimide based color filter whichalso functions as an orientation film.

The aforesaid discussed prior art which utilizes both retarders andcolor filters all utilize one element which functions as a color filterand another separate element which functions as a retarder. It wouldsolve a long-felt need in the art if these two functions could beperformed by a single element which functioned as both a color filterand a retardation element.

The term “interior” when used herein to describe a surface or side of anelement, means the side or surface closest to the liquid crystalmaterial.

The term “retardation value” as used herein means “d·ΔN” of theretardation film or plate, wherein “d” is the film thickness and “ΔN” isthe film birefringence. Defined values may be either positive ornegative depending on the birefringence of the film.

The terms “clockwise” and “counterclockwise” mean as viewed from theobserver's side of the liquid crystal display.

The term “first” when used herein but only as it is used to describesubstrates, polarizers, electrodes, buffing zones, and orientation filmsmeans that the described element is on the incident light side of theliquid crystal material, or in other words, on the side of the liquidcrystal material opposite the viewer.

The term “second” when used herein but only as it is used to describesubstrates, polarizers, electrodes, buffing zones, and orientation filmsmeans that the described element is located on the viewer side of theliquid crystal material layer.

The “horizontal viewing angles” (or X_(ANG)) and “vertical viewingangles” (or Y_(ANG)) illustrated and described herein (see FIG. 24) maybe transformed to conventional LCD angles φ and Θ by the followingequations:

TAN (X_(ANG))=COS (φ)·TAN (Θ)

SIN (Y_(ANG))=SIN (Θ)·SIN (φ)

COS (Θ)=COS (Y_(ANG))·COS (X_(ANG))

TAN (φ)=TAN (Y_(ANG))÷SIN (X_(ANG))

FIG. 24 illustrates the relationship between the four different angles.

It is apparent from the above that there exists a need in the art for amulti-colored liquid crystal display pixel wherein the multi-gap need toadjust the cell gap “d” for each color is eliminated and each colorwavelength is compensated for, thereby improving the viewing angle andcontrast ratios associated therewith for each particular color andsubstantially eliminating different viewing leakages for differentcolors at various viewing angles. There also exists a need in the artfor a single element which functions as both an optical retarder and acolor filter.

SUMMARY OF THE INVENTION

Generally speaking, this invention fulfills the above-described needs inthe art by providing a twisted nematic pixel for use in a liquid crystaldisplay, the pixel comprising: a first subpixel having a first colorfilter and a first retardation film; a second subpixel having a secondcolor filter and a second retardation film; and wherein the first andsecond retardation films have retardation values different from oneanother and the first and second color filters are different from oneanother.

In certain preferred embodiments of this invention, the first and secondretardation films and their respective retardation values are selectedaccording to the color of each subpixel and the pixel further includes athird subpixel having a third color filter and a third retardation filmhaving a retardation value different than the retardation values of thefirst and second retardation films.

In still further preferred embodiments of this invention, the first andsecond retardation films are directly deposited onto the first andsecond color filters respectively and the color filters are located on asubstrate and wherein the substrate is located between the color filtersand a polarizer. In certain other preferred embodiments of thisinvention, the pixel further comprises a retardation layer between thesubstrate and the polarizer, wherein the retardation layer has asubstantially constant retardation value.

In additional preferred embodiments of this invention, the pixel furthercomprises a liquid crystal layer having a thickness less than about 10μm.

In still further preferred embodiments of this invention, the first andsecond retardation films are made of the same material and havesubstantially different thicknesses and wherein the first and secondretardation films are spin-coated onto the first and second colorfilters respectively.

In certain other preferred embodiments of this invention, the firstcolor filter and the first retardation film are combined into a singleintegrally formed polymer based element which functions as both a colorfilter and a retarder, and wherein the single integrally formed polymerbased element is formed by dissolving or immersing a color filter dyeinto a soluble polymer, thus creating a single integrally formed elementwhich functions as both a color filter and a retarder.

In certain further preferred embodiments of this invention, the firstand second retardation films have optical axes which are substantiallyparallel to one another.

In still further preferred embodiments of this invention the first andsecond retardation films have optical axes which are not substantiallyparallel to one another and are selected in accordance with the colorwavelength of each subpixel. In certain further preferred embodiments ofthis invention, the first color filter is a red color filter, the secondcolor filter is a green color filter and the retardation value of thefirst retardation film is about 250-350 nm and the retardation value ofthe second retardation film is about 225-325 nm. In certain otherpreferred embodiments of this invention, the retardation value of thefirst retardation film is about 300-325 nm and the retardation value ofthe second retardation film is about 265-285 nm, and wherein the pixelfurther comprises a liquid crystal layer having a thicknesssubstantially matched to the first transmission minimum of the colorwavelength of the second subpixel.

This invention further fulfills the above described needs in the art byproviding a pixel for use in a liquid crystal display, comprising: afirst subpixel having a first retardation means including a firstoptical axis; a second subpixel having a second retardation meansincluding a second optical axis; and wherein the first optical axis andthe second optical axis are oriented in different directions.

In certain preferred embodiments of this invention, the first and secondretardation means include a retardation film.

In certain further preferred embodiments of this invention, the firstand second retardation means are at least substantially partiallyco-planar, and wherein the different directions are selected inaccordance with the color of each subpixel.

In certain further preferred embodiments of this invention, the pixelfurther comprises first and second transparent substrates and a thirdsubpixel, and wherein said first and second retardation means aredisposed between said first and second substrates. In certain otherpreferred embodiments of this invention, the first optical axis of thefirst retardation means is oriented in a direction at least about 2°different than the orientation of the second optical axis of the secondretardation means.

In still further preferred embodiments of this invention, the directionof the second optical axis is substantially parallel to the buffingdirection of a first orientation means disposed on a first side of aliquid crystal layer, wherein light is adapted to first enter the liquidcrystal layer at the interface between the liquid crystal material andthe first orientation means.

This invention further fulfills the above-described needs in the art byproviding a multi-colored pixel for use in a liquid crystal display,comprising: a first polarizer on a first substrate; a second polarizeron a viewer side of the pixel, and on a second substrate; a liquidcrystal layer disposed between the first and second polarizers; firstand second subpixels each having a different color optical filtertherein for transmitting a different predetermined color or wavelengthof optical radiation; and wherein the first subpixel includes a firstretardation means having a first predetermined retardation value, andthe second subpixel includes a second retardation means having a secondpredetermined retardation value different than the first predeterminedretardation value, and wherein the first and second retardation meansare disposed between the first and second substrates.

In certain further preferred embodiments of this invention, the firstand second retardation values are selected according to the differentcolors of the first and second subpixels and the first subpixel has ared color filter and the second subpixel has a green color filter. Instill further preferred embodiments of this invention, wherein theliquid crystal layer is of the twisted nematic type and is disposedbetween the first and second substrates, and wherein the color filtersare located on the second substrate with the liquid crystal layerdisposed between the color filters and the first substrate.

In certain other preferred embodiments of this invention, the pixelfurther comprises a transparent electrode film located on the first andsecond retardation means whereby the color filters and the retardationmeans are disposed between the electrode and the second substrate, andan orientation film laminated onto the transparent electrode whereby theliquid crystal layer is disposed between the orientation film and thefirst substrate.

In certain further preferred embodiments of this invention, the firstretardation means includes an optical compensating or retardation filmhaving a first thickness and the second retardation means includes anoptical compensating or retardation film having a second thicknessdifferent than the first thickness wherein the second retardation meanshas a retardation value larger than the first retardation means.

In still other preferred embodiments of this invention, the retardationvalues of the retardation means are negative and the first and secondpolarizers are crossed thereby creating a normally white pixel. Incertain further preferred embodiments of this invention, the retardationvalues of the retardation films are positive and the first and secondpolarizers are parallel thereby creating a normally black pixel.

In certain further preferred embodiments of this invention, the liquidcrystal material has about a 90° twist in the OFF state. In certainfurther preferred embodiments of this invention, the first and secondretardation means includes only one optical compensating or retardationfilm and wherein the optical compensation film has an upper terracedsurface defining different thicknesses of the film. In certain otherpreferred embodiments of this invention, the pixel further comprises athird retardation means laminated between the second substrate and thesecond polarizer.

In certain preferred embodiments of this invention, the pixel is anormally black pixel. In other certain preferred embodiments of thisinvention, the pixel is a normally white pixel.

In certain further preferred embodiments of this invention, the firstand second retardation films have positive birefringent values. In stillfurther preferred embodiments of this invention, the first and secondretardation films have negative birefringent values.

This invention further fulfills the above-described needs in the art byproviding a liquid crystal display including a plurality of pixels, thepixels comprising: first and second polarizers with a liquid crystallayer disposed therebetween with orientation means disposed immediatelyadjacent both sides of the liquid crystal layer; means for applying avoltage across the liquid crystal layer; and a single polyimide basedelement which functions as both a color filter and an optical retarder.

In certain preferred embodiments, the polyimide based element includes acolor dye immersed or dissolved therein, and the polyimide is an organicsolvent soluble polyimide or co-polyimide.

In certain further preferred embodiments of this invention, thepolyimide is an organic solvent soluble homopolyimide.

In certain preferred embodiments of this invention, the homopolyimide isselected from the groups consisting of: (i) a pyromellitic dianhydride(PMDA) and 2, 2′-bis (trifluoromethyl) benzidine (BTMB); (ii) 3, 3′, 4,4′-benzophenone tetracarboxylic dianhydride (BTDA) and 2, 2′ bis(trifluoromethyy) benzidine (BTMB); (iii) 4, 4′-oxydiphthalic anhydride(ODPA) and 2, 2′ bis (trifluoromethyl) benzidine (BTMB); (iv) 3, 3′, 4,4′-diphenylsu tetracarboxylic dianhydride (DSDA) and 2, 2′ bis(trifluoromethyl) benzidine (BTMB); (v) 3, 3′, 4, 4′-biphenyltetracarboxylic dianhydride (BPDA) and 2, 2′ bis (trifluoromethyl)benzidine (BTMB); and (vi) 2, 2′-bis (dicarbonylphenyl)hexafluoropropane dianhydride (6FDA) and 2, 2′ bis (tri-fluoromethyl)benzidine (BTMB).

This invention further fulfills the above-described needs in the art byproviding a method of making a liquid crystal display pixel including apolyimide based element which functions in the liquid crystal displaypixel as both a color filter and a retarder, comprising the steps of: a)selecting an organic solvent soluble polyimide selected from the groupconsisting of: (i) a homopolyimide; and (ii) a co-polyimide; b)immersing a color dye therein; and c) positioning said resultingpolyimide based element in a pixel of a liquid crystal display therebyallowing the polyimide based element to function as both a color filterand a retarder in the pixel.

This invention will now be described with respect to certain embodimentsthereof, accompanied by certain illustrations, wherein:

IN THE DRAWINGS

FIG. 1 is a prior art cross sectional view of a conventional normallyblack liquid crystal display pixel of the multi-gap type including tworetardation plates each having a constant birefringent and retardationvalue throughout the entire pixel and wherein the cell gap “d” of the LCmaterial for each subpixel is matched to the first transmission minimumof the color wavelength of that subpixel.

FIG. 2 is a schematic diagram illustrating the optical orientations ofthe prior art components of the twisted nematic NB liquid crystaldisplay pixel of FIG. 1.

FIG. 3 displays the percent transmission of red, green, and blue lightthrough a liquid crystal pixel as a function of cell thickness “d” forthe three wavelengths of red, green, and blue. This graph illustratesthe rationale behind the multi-gap configuration of the pixel shown inFIG. 1.

FIG. 4 is a prior art cross sectional view of a normally black liquidcrystal display pixel including red, green, and blue subpixels. Thispixel includes a single retardation film having a constant birefringentand retardation value throughout the entire pixel wherein the cell gap“d” of the pixel is constant for all three subpixels.

FIG. 5 is a computer simulation graph illustrating the contrast ratiosfor the color red resulting from the prior art pixel shown in FIG. 4,absent the retardation film.

FIG. 6 is a computer simulation graph of the contrast ratios of thecolor green resulting from the prior art pixel shown in FIG. 4, absentthe retardation film.

FIG. 7 is a computer simulation graph illustrating the contrast ratiocurves of the color blue resulting from the prior art pixel shown inFIG. 4, absent the retardation film.

FIG. 8 is a computer simulation graph illustrating the contrast ratiocurves of the color red resulting from the prior art pixel shown in FIG.4, and wherein the retardation film has a retardation value of 275 nm.

FIG. 9 is a computer simulation graph illustrating the contrast ratiocurves of the color green resulting from the prior art pixel shown inFIG. 4, wherein the retardation film has a retardation value of 275 nm.

FIG. 10 is a computer simulation graph illustrating the contrast ratiocurves of the color blue resulting from the prior art pixel shown inFIG. 4, wherein the retardation film has a retardation value of 275 nm.

FIG. 11 is a schematic diagram of the optical components of a firstembodiment of an NB twisted nematic liquid crystal display pixelaccording to this invention. The first and second linear polarizers havetransmission axes parallel to one another in direction A₀. The liquidcrystal pixel has a first buffing zone having a direction B₁substantially perpendicular to the transmission axes of the polarizers.The direction A₀ of the second buffing zone is substantially parallel tothe transmission axes of the polarizers. The color filters and colorpersonalized or patterned retardation films are disposed between thesecond buffing zone and the second or exit polarizer.

FIG. 12 is an optical diagram of the personalized blue retarder 208according to the first embodiment of this invention wherein the blueretardation film 208 has its optical axis R_(B) rotated clockwise fromdirection B₀, and wherein direction B₀ is substantially parallel todirection B₁.

FIG. 13 is a computer simulation graph of the contrast ratio curves ofthe color red resulting from the pixel of the first embodiment of thisinvention illustrated in FIGS. 11 and 12 wherein the personalized redretardation film has a retardation value of 315 nm.

FIG. 14 is a computer simulation graph illustrating the contrast ratiocurves of the color green resulting from the pixel of the firstembodiment of this invention illustrated in FIGS. 11 and 12 wherein thepersonalized green retardation film has a retardation value of 275 nm.

FIG. 15 is a computer simulation graph of the contrast ratio curves ofthe color blue resulting from the pixel of the first embodiment of thisinvention illustrated in FIGS. 11 and 12 wherein the personalizedretardation film for the color blue has a retardation value of 240 nm.

FIG. 16 is a cross sectional view of a second embodiment of a twistednematic liquid crystal display pixel according to the present invention.This second embodiment utilizes a personalized retardation film for eachcolor subpixel, each film having a preselected retardation value andoptical orientation selected in accordance with the color or wavelengthof its subpixel. Each subpixel (e.g., red, green, and blue) of thissecond embodiment has a personalized or patterned retardation film madeof the same material, but of different thicknesses thereby creatingdifferent retardation values matched to the particular wavelength ofeach subpixel. The personalized retardation films of this embodiment arelocated on the interior surfaces of the color filters.

FIG. 17 is a cross sectional view of a third embodiment of a twistednematic liquid crystal pixel of this invention. This third embodimentutilizes personalized or patterned retardation films only in the red andgreen subpixels, with the blue subpixel free of any such film.Furthermore, the third embodiment is provided with a retardation film orlayer having a constant birefringent and retardation value between thesecond substrate and the second polarizer.

FIG. 18 is a cross sectional view of a fourth embodiment of a twistednematic liquid crystal display pixel according to this invention. Thepersonalized retardation film of the fourth embodiment has an upperterraced surface thereby defining different thicknesses and differentretardation values for the retardation film in each subpixel.

FIG. 19 is a cross sectional view of a fifth embodiment of a liquidcrystal display pixel according to this invention. In the fifthembodiment, each subpixel has a personalized retardation film whereinthe film of each subpixel is made of a different material preferablyhaving a different birefringent and thus retardation value. Thethicknesses of the retardation films in the fifth embodiment may besubstantially equal or, alternatively, may be substantially differentdepending upon the birefringent values and required thicknesses of eachsubpixel.

FIG. 20 is a cross sectional view of a sixth embodiment of a twistednematic liquid crystal display pixel according to this invention. Thesixth embodiment utilizes personalized retardation films located on theinterior surface of the first substrate and an optical color filter foreach subpixel located on the interior surface of the secondsubstrate-whereby the color filters and retardation films are disposedon opposite sides of the liquid crystal layer.

FIG. 21 is a cross sectional view of a seventh embodiment of a twistednematic liquid crystal display pixel according to the present invention.The color personalized or patterned retardation films of the seventhembodiment are disposed on the exterior surfaces of the color filtersthereby being located between the color filter of each subpixel and thesecond substrate thereof.

FIG. 22 is a cross sectional view of an eighth embodiment of thisinvention wherein a polymer or polyimide based film in each subpixelacts as both a color filter and a retarder.

FIG. 23 is a partial cut-away view of an LCD including a plurality ofpixels.

FIG. 24 is a graph illustrating the angular relationship between thehorizontal and vertical angles discussed herein, and the conventionalLCD angles φ and Θ.

DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS OF THIS INVENTION

FIG. 11 is a schematic view of an arrangement of a first embodiment of aliquid crystal display pixel according to this invention. The rear“X-buffed” normally black pixel shown in FIG. 11 includes a firstpolarizer 202, a first buffing or orientation film 204, a secondorientation film 206, a personalized uniaxial blue retardation film 208,a personalized uniaxial green retardation film 210, a personalizeduniaxial red retardation film 212, a blue color filter 214, a greencolor filter 216, a red color filter 218, and finally a second or exitpolarizer 220. The retardation films of this first embodiment havepositive birefringent and retardation values and are uniaxial. Suchretardation films are commercially available and may be obtained from,for example, Nitto Corp., Japan, or Nitto Denko America, Inc., NewBrunswick, N.J. Nitto supplies, for example, 315 nm retardation filmshaving a model number NRF-RF 315.

A liquid crystal layer (not shown) is sandwiched between the first andsecond orientation films 204 and 206, wherein the liquid crystal layer,with no voltage applied thereto, twists incident light about 82°-100°. Afirst substrate (not shown) is disposed between the first linearpolarizer 202 and the first orientation film 204, with a first electrodelayer (not shown) being laminated between the first substrate (notshown) and the first orientation film 204. A second substrate (notshown) is preferably sandwiched between the color filters and the secondlinear polarizer 220. Furthermore, a second electrode layer (not shown)is preferably disposed between the second orientation film 206 and thepersonalized red, green, and blue retardation films.

The first and second linear polarizers 202 and 220 have transmissionaxes which are parallel to one another in a direction A₀ therebydefining a normally black pixel. The first orientation film 204 isoriented or buffed in a direction substantially perpendicular to thetransmission axes of the linear polarizers 202 and 220, thereby defininga first buffing zone for orienting the molecules of the liquid crystallayer adjacent the orientation film 204 in a direction B₁. The secondorientation film 206 has a buffing or orientation directionsubstantially perpendicular to direction B₁ and substantially parallelto direction A₀, thereby orienting the liquid crystal molecules adjacentthe film 206 in a direction parallel to the transmission axes of thelinear polarizers 202 and 220. The liquid crystal material of thisparticular embodiment is left handed in that it twists light passingtherethrough in the clockwise direction.

The liquid crystal layer (not shown) of this first embodiment shown inFIG. 11 has a thickness of about 5.70 micrometers (μm) and is matched tothe first transmission minimum of the green wavelength at 550 nm.Therefore, the thickness “d” of the liquid crystal layer is less thanthe first transmission minimum of the red color wavelength at 630 nm,and is greater than the first transmission minimum of the bluewavelength at 480 nm.

The normally black pixel of this first embodiment behaves optically inthe following manner. Normally incident white light 201, including blue,green, and red wavelengths, first proceeds through the first linearpolarizer 202 and is linearly polarized in direction A₀. After the light201 is polarized by the first polarizer 202, it proceeds through thefirst transparent substrate (not shown) and the first electrode layer(not shown). After propagating through the first electrode layer, thelinearly polarized light enters the liquid crystal layer (not shown) asordinary light (as opposed to extraordinary light). The light enteringthe liquid crystal material is “ordinary light” because the firstbuffing zone of orientation film 204 is substantially perpendicular tothe transmission axis of the first polarizer 202.

When the light reaches the first orientation film 204, it is polarizedin direction A₀ which is substantially perpendicular to the buffingdirection B₁ of the first orientation film 204. Due to the perpendicularorientation of the buffing directions of the first and secondorientation films 204 and 206, the light is twisted by the OFF stateliquid crystal material to an extent of about 82°-100° as it proceedstherethrough. As can be seen from the buffing directions of the firstand second orientation films 204 and 206, the polarization direction ofthe red, green.; and blue light is twisted clockwise by the left handedliquid crystal material and is polarized in a direction B₀ plus or minusabout 8° or less when it reaches the exit end of the liquid crystalmaterial and proceeds through the second orientation film 206.

Because the cell gap “d” of the liquid crystal material is matched tothe first transmission minimum of the wavelength of the color green, thenormally incident green wavelength of light which passes through theliquid crystal material is twisted about 90° in the clockwise directionand is polarized in direction B₀ when it reaches the green personalizedretardation film 210. Because the cell gap “d” is greater than the firsttransmission minimum for the color blue at a wavelength of about 480 nm,the normally incident blue light is twisted by the liquid crystalmaterial to an extent substantially greater than 90° (e.g. about92°-100°). If not for the blue retardation film of this embodiment, theovertwisted portion of the blue light would leak through the secondpolarizer when the pixel was in the OFF state. Accordingly, the bluewavelength of light, when it reaches the personalized or patterned blueretardation film 208, is not polarized in direction B₀, but is polarizedin a direction P_(B) which is rotated clockwise relative to directionB₀.

Likewise, because the liquid crystal material cell gap “d” is less thanthe first transmission minimum for the color red, normally incidentlight at the red wavelength of about 630 nm is only twisted about82°-88° by the liquid crystal material and therefore is not polarized ina direction B₀ when it reaches the red personalized retardation film212. The red light is instead polarized in a direction P_(R) which isrotated counterclockwise relative to direction B₀, when it reaches thepersonalized red retardation film 212.

The incident light, after being retarded by the color personalizedretardation films, then proceeds through the blue, green, and red colorfilters 214, 216, and 218. Finally, after passing through the colorfilters, the incident light reaches the second or exit polarizer 220adjacent the viewer which has a transmission axis parallel to that ofthe first linear polarizer 202. Normally incident light, of the blue,green, and red wavelengths, will at this point, assuming the pixel is inthe OFF state, be substantially polarized in a direction B₀ and willtherefore be absorbed by the second linear polarizer 220, therebycreating a darkened or OFF state pixel thereby substantially eliminatingdifferent color leakage at different viewing angles.

Data is displayed through the NB pixel of this first embodiment, as inconventional NB twisted nematic liquid crystal display pixels, byselectively transmitting a variable voltage across the liquid crystalmaterial. When a voltage is applied across the liquid crystal material,the LC molecules are aligned in the direction of the electric field inaccordance with the strength of the voltage, and the twist effect doesnot occur, thereby giving rise to light polarized in the direction A₀when it reaches the second linear polarizer 220. Light polarized indirection A₀ when it reaches the second or exit linear polarizer 220will be transmitted therethrough creating a color display.

The color personalized or patterned retardation films 208, 210, and 212of this first embodiment all have different retardation values (d·ΔN).The retardation value of each color personalized retardation film 208,210, and 212 of this particular embodiment is selected by providing eachcolor subpixel with a half wave plate retarder. In other words, becausethe red wavelength is 630 nm, the retardation value of the personalizedred retardation film 212 will be about 315 nm. Likewise, because thegreen wavelength is 550 nm, the personalized green retardation film 210will have a retardation value of about 275 nm. Also, because the bluecolor wavelength is 480 nm, the personalized or patterned blueretardation film 208 will have a retardation value of about 240 nm.Therefore, the retardation film for each colored subpixel has adifferent retardation value chosen in accordance with the colorwavelength of the subpixel.

In this particular embodiment, the blue retardation film 208 has thelowest retardation value at about 240 nm, and the red color personalizedretardation film 212 has the highest retardation value at about 315 nm.However, in other particular embodiments of this invention, this neednot be the case, and, for example, the blue personalized retardationfilm may have a higher retardation value than the green and redretardation films. Furthermore, as will be clear to those of ordinaryskill in the art, the different embodiments of this invention may beused in conjunction with any combination of colors, not just red, green,and blue.

Another important aspect of this invention is the orientation of theoptical axis of each of the color personalized retardation films 208,210, and 212. In this particular embodiment of this invention, the greenpersonalized retardation film 210 has its optical axis R_(G) oriented ina direction substantially parallel to direction B₀ because the cell gap“d” of the liquid crystal layer is matched to the first transmissionminimum of the green wavelength and the green wavelength of light istherefore twisted about 90° by the liquid crystal layer.

However, because the cell gap of the liquid crystal material is greaterthan the first transmission minimum for the blue wavelength, and theblue light is twisted to an extent greater than about 90°, thepersonalized blue retardation film 208 has its optical axis oriented ina direction R_(B) which is half-way between the polarization directionP_(B) of the blue light when it reaches the retardation film 208 and thedirection B₀ which is parallel to the first buffing direction B₁. Byorienting the optical axis R_(B) of the blue personalized retardationfilm 208 in a direction about half-way between directions P_(B) and B₀and providing the personalized blue retardation film 208 with aretardation value equal to about one-half the blue wavelength, theso-called over-twisting of the blue light by the liquid crystal materialis compensated for and the retardation film 208 acts to shift thepolarization direction of the blue light from direction P_(B) back intoa direction substantially parallel to direction B₀ after the lightproceeds through the film 208.

In this particular first embodiment of this invention, the liquidcrystal material twists the blue wavelength about 100°, and thereforethe orientation axis R_(B) of the blue retardation film 208 is orientedin a direction about 5° clockwise relative to direction B₀. Therefore,as a result of the correcting nature of the blue retardation film 208,the polarization directions of both the blue and green light aresubstantially parallel to one another as they enter the blue and greencolor filters 214 and 216.

Likewise, because the liquid crystal layer thickness “d” is less thanthe first transmission minimum of the red wavelength at 630 nm, normallyincident red light is twisted to an extent less than about 90° (e.g.about 82°-88°) by the liquid crystal material as it passes therethrough.The polarization direction P_(R) of the red light when it reaches thepersonalized retardation film 212 is therefore rotated or orientedcounterclockwise relative to direction B₀. The red retardation film 212,having a retardation value of about 315 nm, has its optical axis R_(R)oriented in a direction half-way between the red light polarizationdirection P_(R) as it exits the liquid crystal material and thedirection B₀ which is substantially parallel to the buffing direction B₁of the first orientation film 204. In this particular embodiment of thisinvention, the pre-retardation actual polarization direction P_(R) ofthe red light is about 8° counterclockwise relative to direction B₀, andthe retardation film optical axis direction R_(R) is about 4°counterclockwise relative to the direction B₀.

By providing the red personalized or patterned retardation film 212 withan appropriate half-wave retardation value (315 nm in this embodiment)and an optical axis R_(R) orientated half-way between directions B₀ andP_(R), the polarization of the red wavelength is shifted by theretardation film 212 to a direction substantially parallel to B₀ as itexits the personalized red retardation film 212.

Therefore, the polarization directions of each of the blue, green, andred wavelengths are substantially parallel to one another as they exittheir respective personalized retardation films and enter their colorfilters 214, 216, and 218. This is accomplished, as described above, byproviding each personalized retardation film with a retardation valuechosen in accordance with the wavelength of each color, and orientingthe optical axes of the respective retardation films in appropriatedirections as discussed above. By properly selecting personalized valuesfor the different retardation films of this embodiment and orientingtheir respective optical axes in accordance with the wavelength of eachsubpixel, one can nearly eliminate different color leakages at differentviewing angles. This is accomplished by substantially lining up theviewing zones of each subpixel in the same viewing angle areas as shownin below-discussed FIGS. 13-15.

As will be realized by those skilled in the art, the cell gap “d” neednot be matched to the first transmission minimum of a subpixel color. Insuch a case, the personalized retardation films could be arranged tocompensate for such an arrangement in accordance with the teachings ofthis invention.

Of course, the first embodiment of this invention could also bepracticed with right handed liquid crystal material which twists thelight in the counterclockwise direction as it passes therethrough. Insuch a case, the directions R_(R) and P_(R) would be oriented clockwiserelative to B₀, and directions R_(B) and P_(B) would be orientedcounterclockwise to direction B₀. In other words, the optical axes ofthe red and blue retardation films would be substantially mirroredsymetrically about the B₀ axis or direction.

Furthermore, the first embodiment of this invention would also produceexcellent results if the first and second polarizers were each rotatedabout 90° in either direction, thereby defining a front “X-buffed” NBpixel.

FIG. 12 is a close-up view of the personalized blue retardation film 208of the first embodiment shown in FIG. 11. As can be seen in FIG. 12,direction B₀ is parallel to the buffing direction B₁ of the firstorientation film 204 and is perpendicular to the transmission axesdirections A₀ of the linear polarizers 202 and 220. Direction B₀ is alsoparallel to the green polarization direction as it exits the liquidcrystal material and reaches its personalized retardation film 210 andthe green retardation axis R_(G). Direction P_(B) is the actualpolarization direction of the blue light as it exits the liquid crystalmaterial and reaches the personalized blue retardation film 208. Inaccordance with the first embodiment of this invention, the optical axisR_(B) of the blue personalized retardation film 208 is chosen to beoriented in a direction half-way between directions B₀ and P_(B).Orientation of the blue retardation film axis R_(B) in this directionacts to shift the polarization direction of the blue light as it passesthrough the retardation film 208 from direction P_(B) to a directionsubstantially parallel to direction B₀ as it exits the blue retardationfilm 208 and proceeds towards the blue color filter 214.

The optical orientation and values of the elements described in thefirst embodiment of this invention may, of course, be used incombination with the structural arrangements described in other certainembodiments of this invention (e.g. the second, fourth, fifth, sixth,seventh, and eighth embodiments described herein).

FIGS. 13-15 are computer simulation graphs which illustrates thecontrast ratios resulting from the first embodiment of this inventionillustrated in FIGS. 11 and 12.

FIG. 13 illustrates the contrast ratio curves for the red wavelength of630 nm wherein the cell gap “d” is 5.70 micrometers (μm) and matched tothe first transmission minimum of the color green at a wavelength of 550nm. This graph also uses parameters including an ON voltage of 4.0volts, an OFF voltage of 0.2 volts, a personalized red retardation filmhaving a retardation value of 315 nm, and a red retardation film axisdirection R_(R) rotated or oriented 4° in the counterclockwise directionrelative to direction B₀. As can be seen by the contrast ratio graph forthe color red shown in FIG. 13, the contrast ratio at normal (0°vertical, 0° horizontal) is about 110-120:1. Along the horizontal 0°axis, the 30:1 contrast ratio curve extends upward to a vertical viewingangle of greater than 40° and downward to a vertical viewing angle ofabout −33°. This is, of course, a significant improvement over thecontrast ratio curves for the color red illustrated in FIGS. 5 and 8.The improvement is a result of the red personalized retardation filmincluding its retardation value and orientation axis of the firstembodiment of this invention. As can also be seen by the red contrastratio graph illustrated in FIG. 13, the contrast ratio along the 0°vertical axis is greater than 100:1 through horizontal angles of both−60° and +60°. This also is a significant improvement over the redcontrast ratio curves illustrated in FIGS. 5 and 8.

FIG. 14 illustrates the contrast ratio curves for the green wavelengthof 550 nm resulting from the first embodiment of this invention shown inFIG. 11 and 12. The FIG. 14 computer simulation graph utilizesparameters including a cell gap of 5.70 micrometers, a personalizedgreen retardation film having a retardation value of 275 nm, apersonalized green retardation film optical axis direction R_(G)extending in a direction B₀ which is perpendicular to the transmissionaxes direction A₀ of the two linear polarizers 202 and 220, a driving ONvoltage of 4.0 volts, and an OFF voltage of 0.2 volts. The graph of FIG.14 is similar to that illustrated in FIG. 9 because the cell gap of 5.70micrometers of the liquid crystal display of the first embodiment ismatched to the first transmission minimum of the color green.

However, it will be understood by those of ordinary skill in the artthat the cell gap need not be matched to the first transmission minimumof any colored subpixel. In such a case, one way in which to compensatefor such is to provide for the orientation of the personalizedretardation film optical axes of this first embodiment to be rotated inorder to compensate for the non-matching cell gap.

As can be seen in FIG. 14, the contrast ratio of the green wavelength inthe first embodiment of this invention is about 270:1 at normal, withthe 30:1 contrast ratio curve extending off the graph along both 0°axes. This is a significant improvement over the green contrast ratiocurve shown in FIG. 6.

FIG. 15 illustrates the contrast ratio curves of the blue wavelengthresulting from the first embodiment of this invention illustrated byFIGS. 11 and 12. The parameters for this graph are the same as thosedescribed with respect to the graphs of FIGS. 13-14. The contrast ratioat normal is about 220:1 for the blue wavelength which is a significantimprovement over the blue wavelength contrast ratios at normal shown inFIGS. 7 and 10. Along the 0° horizontal viewing axis, the 30:1 contrastratio curve extends upward off the graph to an angle greater than 40°,and downward to a vertical viewing angle of about −38°. Along the 0°vertical axis the 30:1 contrast ratio curve extends from the horizontalviewing angles of about −48° to +56°. Again, this is a significantimprovement over the blue contrast ratio curves of FIGS. 7 and 10.

As can be seen from the contrast ratio graphs of FIGS. 13-15, thecontrast ratio curves of all three colors are very good in that they allhave high contrast ratios at normal and the 30:1 contrast ratio curvefor all colors extends horizontally and vertically to significantextents in all directions. This results in a substantial elimination ofdifferent color leakages at different viewing angles, including thenormal viewing angle, because the improved high contrast ratio viewingzones of all three colors are substantially angularly aligned with oneanother. This is a significant improvement over the prior art becauseparticular embodiments of this invention, such as that shown in FIGS. 11and 12, allow one to eliminate the multi-gap need of matching the cellgap “d” to the first transmission minimum of multiple colors, whilestill providing for superior contrast ratio curves for all requisitecolors. This first embodiment eliminates the need of the multi-gapconfiguration shown in FIG. 1 and compensates for the wavelength of eachcolor by personalizing or patterning the retardation values andorientations of personalized retardation films provided for eachsubpixel.

The color wavelengths used in the computer simulations and certainembodiments of this invention are merely illustrative. Those skilled inthe art will readily realize that embodiments of this invention may becarried out using different color wavelengths.

FIG. 16 is a cross sectional view of a second embodiment of a twistednematic liquid crystal display (TNLCD) pixel according to the presentinvention. The TNLCD of this embodiment may be either of the normallyblack or normally white type, depending on the orientation of the belowdiscussed polarizers. The liquid crystal material may twist normallyincident light anywhere in the range of about 80°-270°, but preferablyabout 82°-100°. Optical radiation 30 from a radiation source is appliedto the liquid crystal display pixel.

The applied optical radiation 30 is typically from a single white lightsource which irradiates light through each subpixel of the pixel shownin FIG. 16. The pixel 31 includes a red subpixel, a green subpixel, anda blue subpixel. However, it should be noted that a greater or lessernumber of different colored subpixels may be utilized with the color ofeach subpixel being chosen according to the specific intended use of thepixel. The optical radiation 30 first passes through a first linearpolarizer 32. The optical radiation after passing through the firstlinear polarizer 32 then passes through the first transparent substrate34. The transparent substrate 34 consists essentially of, for example,glass, quartz, plastic, or the like (most preferably glass).

The optical radiation 30 then proceeds through the transparent activematrix 36 which includes pixel electrodes therein. The active matrix 36includes therein an electrode corresponding to each subpixel, asillustrated for example by elements 18 a, 18 b, and 18 c in FIG. 1herein. The active matrix 36 and pixel electrodes therein aretransparent thereby allowing the optical radiation 30 to passtherethrough.

The optical radiation then proceeds into the twisted nematic liquidcrystal layer 38 which is sandwiched between the first transparentsubstrate 34 and a second transparent substrate 40. The thickness “d” ofthe liquid crystal layer 38 is preferably less than about 10 μm and mostpreferably about 5-7 μm, although any thickness “d” may be used inconjunction with this invention. For example, any thicknessessuper-twisted liquid crystal display cell may be used. The liquidcrystal layer 38 of this embodiment preferably twists the polarizedradiation about 82°-100° when the pixel is in the OFF or unenergizedstate. The degree of twist depends on the alignment of the buffing zones(not shown), the thickness “d” of the LC material, and the wavelength oflight being twisted.

On the interior surface of the first substrate 34 is the aforesaidmentioned active matrix 36 preferably including ITO pixel electrodestherein. These electrodes are preferably connected to thin filmtransistors (not shown) arranged in a matrix array for selectivelyenergizing the pixel electrodes. These thin film transistors (not shown)act as switching devices. The electrodes within the active matrix 36selectively apply a variable electric field to the liquid crystalmaterial 38 of each subpixel of the pixel 31 thereby allowing selecteddata images to be displayed. Driving schemes for driving the activematrix LCD of this invention are known throughout the art and aredisclosed, for example, in U.S. Pat. Nos. 4,855,724 and 4,830,468, thedisclosures of which are hereby incorporated herein by reference.

The various embodiments of this invention will also work in combinationwith LCDs driven by diodes, mims, etc., whether or not they are of theactive matrix type. Plasma addressed LCDs may also be used inconjunction with certain preferred embodiments of this invention.

In the second embodiment of this invention, a blue optical filter 42 isprovided in the blue subpixel, a green optical filter 44 is provided inthe green subpixel, and a red optical filter 46 is provided in the redsubpixel. The color filters 42, 44, and 46 are coupled to the interiorsurface of the second glass substrate 40 in this particular embodiment.Deposited on the interior surface of the color filter of each subpixelis a color personalized retardation film 50, 52, and 54 selectedaccording to the wavelength of the color of each subpixel.

The blue subpixel retardation film 50 deposited on the blue opticalfilter 42 has a retardation value and preferably but not necessarily anorientation axis direction selected according to the optical wavelengthof the color blue. The green subpixel retardation film 52 is depositedon top of the green optical filter 44 and has a retardation value and anorientation axis direction preferably chosen in accordance with thecommonly known wavelengths of the color green. The red subpixelretardation film 54 deposited on the red color filter 46 has abirefringent and retardation value, and an orientation axis directionpreferably chosen in accordance with the wavelength of the color red.

In certain embodiments of this invention the orientation axes of thepersonalized retardation films are substantially parallel to oneanother, while in other embodiments they are substantially not parallelto one another as in the first embodiment of this invention.

As will be clear and well known to those of skill in the liquid crystaldisplay art, red, green, and blue colors of optical radiation 30 havedifferent wavelengths (λ). Accordingly, as discussed above, theretardation film of each subpixel is particularly selected in accordancewith the color wavelength of each particular subpixel because the liquidcrystal layer 38 affects or retards each wavelength to a differentextent.

The values and optical axis orientations of the retardation films of thecolor subpixels are chosen so as to compensate for the differentwavelength of each color. This retardation film wavelength compensationeliminates the need for the multi-gap approach of FIG. 1. Accordingly,the personalized retardation films of this invention compensate for thedifferent twisting and retardation of the different wavelengths.

Also, the retardation film 50 of the blue subpixel in certain particularembodiments of this invention has a first retardation value less thanthe values of the retardation films 52 and 54 in the green and redsubpixels. Likewise, the retardation film 52 of the green subpixel incertain particular embodiments of this invention has a secondretardation value different than those of the red and blue subpixels,and the retardation film 54 of the red subpixel in certain particularembodiments of this invention has a retardation value chosen inaccordance with the wavelength of the color red whereby the retardationvalue of the red retardation film 54 is larger than the respectivebirefringent values of the retardation films of the blue and greensubpixels. Therefore, in certain particular embodiments of thisinvention the retardation film of the red subpixel has the largestretardation value while the retardation film 50 of the blue subpixel hasthe smallest retardation value. The retardation value, of course, may bechanged by adjusting the thickness, the birefringence, or both.

As a result of the personalized retardation films, the phase shift ofthe liquid crystal material 38 is substantially matched to thewavelength of each particular color by use of the personalizedretardation films 50, 52, and 54, instead of by matching the thickness“d” of the cell to the first transmission minimum of each color. Bymatching via retarders the phase shift of each subpixel to itsparticular wavelength, the need for the multi-gap configuration (seeFIG. 1) where (d·ΔN)÷λ is matched to the first transmission minimum ofeach particular wavelength by varying the cell gap “d,” is eliminated.By following the teachings of this invention, the Gooch-Tarry principlesof matching (d·ΔN)÷λ to the first transmission minimum of eachwavelength need no longer be followed in order to achieve superiorcontrast ratios for a plurality of wavelengths.

The retardation values of each retardation film of the respectivesubpixels in certain embodiments of this invention may be varied byusing one material for all retardation films 50, 52, and 54, and varyingthe thickness thereof to create different retardation values. As thethickness of a retardation film of a selective material increases, sodoes the retardation value of the film. Therefore, one needs simply tothicken a particular retardation film in order to increase itsretardation value.

Alternatively, the retardation values of the retardation films 50, 52,and 54 in certain embodiments of this invention may be changed by usingdifferent materials. It is known that different retarding materials havedifferent birefringent values. Therefore, a first material could be usedfor the retardation film 50 of the blue subpixel, a second material forthe retardation film 52 of the green subpixel, and a third material forthe retardation film 54 of the red subpixel, wherein the first, second,and third materials all have different birefringent and/or retardationvalues selected in accordance with the color of each subpixel.

As stated above, the thickness of each retardation film or layer dependsupon the required birefringent value of that particular film which is tobe selected in accordance with the wavelength of that particularsubpixel.

With respect to the materials to be used for the retardation films ofthis invention, both positive and negative birefringent retarders areknown in the art and both may be used in certain embodiments of thisinvention. U.S. Pat. No. 4,138,474, hereby incorporated herein byreference, discloses multiple positive and negative birefringentretardation films which may be used as the retardation films of thisinvention. Also, U.S. Pat. No. 5,071,997, hereby incorporated herein byreference, discloses a class of soluble polyimides and co-polyimidesmade from substituted benzidines and aromatic dianhydrides and otheraromatic diamines which may be used in forming retardation films withnegative birefringence. The polyimides of U.S. Pat. No. 5,071,997 whichare soluble may be spin coated directly onto the color filters or otherappropriate layers in certain embodiments of the instant invention. Thisspin-coating is preferably used in combination with conventional LCDmanufacturing techniques such as photolithography. Alternatively,capillary-coating could be used instead of spin-coating.

Deposited on the interior surface of the retardation films 50, 52, and54 shown in FIG. 16 is a transparent conductive electrode (not shown)which acts as the second electrode for each color component or subpixelof the pixel 31. A power supply (not shown) is provided to apply apotential to the liquid crystal material 38 which occupies the regionbetween the electrodes of the matrix 36 and the electrode layer (notshown) deposited on top of the retardation films. As will be clear andwell-known to those skilled in the liquid crystal display art, the powersupply (not shown) is typically used in conjunction with knownaddressing circuitry (not shown) for selectively applying apredetermined voltage to each of the color component unit or subpixelelectrodes. In this manner, an image can be displayed by energizingselective subpixels and/or pixels.

The optical radiation, having been linearly polarized by the firstpolarizer 32, is rotated during transmission through the liquid crystalmaterial 38 between the opposing electrodes. The twisted nematic cell 38of this embodiment preferably rotates or twists the light about82°-100°, although any degree of twist within a LC cell may be used incertain embodiments of this invention.

The optical radiation, after transmission through the liquid crystalmaterial 38 with each color wavelength being retarded and/or twisted toa different extent, passes through the lower electrode (not shown), thecolor personalized retardation films 50, 52, and 54, and the opticalcolor filters 42, 44, and 46. The optical filters select the colorcomponents to be transmitted by each subpixel of the liquid crystalpixel 31. After transmission through the color filters, the opticalradiation is transmitted through the second transparent substrate 40 andfinally, through the second or exit linear polarizer 56. After beingtransmitted through the second polarizer 56, the radiation istransmitted to an observer viewing the display.

Black matrix or shielding units 41 in certain embodiments of thisinvention are positioned between the color filters and are opaque tolight thereby preventing unfiltered light from being transmitted throughthe pixel.

Both normally white and normally black liquid crystal displays mayutilize certain embodiments of the present invention. In other words,the transmission axes of the polarizers 32 and 56 may be crossed incertain embodiments, thereby creating a normally white liquid crystaldisplay pixel when the liquid crystal material 38 has about aconventional 90° twist. Alternatively, in certain embodiments of thisinvention the linear polarizers 32 and 56 may have their transmissionaxes arranged parallel to one another, thereby creating a normally blackliquid crystal display pixel when the liquid crystal material 38 hasabout a conventional 90° twist in the OFF state. The orientation of thepolarizers dictates the retardation value and orientation of retardationfilms to be used in the display. Also, it should be evident to thoseskilled in the art that super twisted (e.g. twisted angles of 90°-270°)liquid crystal displays, ECB displays, and homeotropic displays may alsoutilize the concepts set forth in particular embodiments of thisinvention.

When the optical radiation is transmitted at oblique angles through thepixel 31, the off axis transmission becomes increasingly ellipticallypolarized with angle, a result of the birefringence of the liquidcrystal material. The birefringence of the liquid crystal materialaffects differently each particular wavelength of light (e.g. red,green, and blue wavelengths). The result of this elliptical polarizationis non-uniformity of radiation contrast ratios as a function of angleabout the normal axis after transmission of the radiation through theliquid crystal material 38. In order to compensate for thenon-uniformity, the personalized retardation films 50, 52, and 54 are,in the second embodiment, interposed between the liquid crystal material38 and the second substrate 40 as shown in FIG. 16. The presence of theretardation films results in a decrease in the elliptical polarizationof the radiation applied to the second polarizer 56. Consequently, theuniformity of the radiation contrast ratios transmitted through thelinear polarizing plate 56 is improved.

Because the birefringence of the liquid crystal material 38 affectsdifferently each wavelength, the birefringence of each retardation film50, 52, and 54 is personalized according to the color or wavelength ofeach subpixel. Accordingly, as a result of the personalization of therespective retardation films 50, 52, and 54, the leakage of each color(e.g., red, green, and blue) is substantially the same, one colorrelative to the others, throughout all viewing angles. Therefore, as aresult of the personalization or patterning of the retardation films ofeach particular subpixel according to the wavelength of each subpixel,there is no longer the problem of having different relative colorleakages at different viewing angles.

As should be evident from the above, the personalizing or patterning ofthe retardation films according to the color of each subpixel as taughtby this invention satisfies two long felt needs in the liquid crystaldisplay art. First, the personalization of the retardation filmsaccording to color improves the contrast ratio of each color at certainviewing angles and prevents excess leakages of one color relative toother colors at particular viewing angles (including the ON axis viewingangle). Secondly, the patterning or personalization of the retardationfilms of this invention compensates for the different wavelengths of thedifferent colors and thereby eliminates the need to follow the multi-gapapproach of matching the parameters (d·ΔN)÷λ of each subpixel to thefirst transmission minimum of each color as taught by, for example, U.S.Pat. No. 4,632,514. As a result, the teachings of aforesaid mentionedU.S. Pat. No. 4,632,514 no longer need be followed in that the phaseshift of the liquid crystal material is matched to the particularwavelength of each subpixel by personalizing the retardation films ofthe respective subpixels. The need for matching the thickness “d” of aliquid crystal material to the first transmission minimum of each coloris eliminated by this invention because the personalized retardationfilms and their respective birefringent values and orientationscompensate for the phase shift resulting from the elimination ofmatching the cell gap “d” to the first transmission minimum of eachwavelength.

Furthermore, as will be recognized by those skilled in the art, theretardation films 50, 52, and 54 of certain embodiments of thisinvention may include one, two, or more layers immediately adjacent oneanother or spaced on opposite sides of the liquid crystal layer as isknown throughout the art. For example, U.S. Pat. Nos. 5,150,235,4,385,806, and 5,184,236, the disclosures of all of which areincorporated herein by reference, teach multi-layered retardation filmswhich may be used within the confines of this invention. Accordingly,all of the retardation films described with respect to certainembodiments of this invention may consist of one, two, or more layersoriented according to the particular use intended for the resultingpixel. Furthermore, both uniaxial and biaxial retardation films may beused in conjunction with particular embodiments of this invention.

FIG. 17 illustrates a third embodiment of this invention. FIG. 17 is across sectional view illustrating a third embodiment of this inventionwherein only the red and green subpixels of the pixel 31 are providedwith retardation films, 80 and 81 respectively, personalized to theparticular wavelength of each subpixel as discussed above. The bluesubpixel is left alone without the company of a correspondingpersonalized retardation film.

However, in this third embodiment another retardation film or layer 60having a constant retardation value is provided beneath the secondtransparent substrate 40 between the substrate 40 and the secondpolarizer 56. This additional retardation film or layer 60 behavesoptically as any other conventional retardation film.

The retardation value of the retardation film 60 is added to theretardation values of the personalized retardation films within eachsubpixel. For example, as shown in the third embodiment illustrated inFIG. 17, if the retardation value of the personalized retardation film80 of the red subpixel was 10 units, and the retardation value of theretardation film 60 located between the second substrate 40 and thepolarizer 56 was 5 units, the total retardation value of the retardationfilms of the red subpixel would be 15 units.

It should also be apparent to those skilled in the art that thepersonalized retardation films in certain embodiments of this inventionmay all be located in a position similar to that of retardation film 60shown in FIG. 17, outside of the substrates. Alternatively, in certainembodiments the personalized retardation films may also be positionedbetween the first polarizer 32 and the first substrate 34. Also, theretardation film 60 as shown in FIG. 5 may alternatively be positionedbetween the first polarizer 32 and the first substrate 34. Like thesecond embodiment, the third embodiment may be either a NB or a NWtwisted nematic LCD, preferably with a cell twist in the OFF state ofabout 82°-100°.

FIG. 18 illustrates a fourth embodiment of this invention. FIG. 18 is across sectional view illustrating a twisted nematic LCD including asingle personalized or patterned retardation film 62 which has aninterior surface which is terraced defining different thicknesses foreach particular subpixel. Alternatively, the exterior surface couldinstead be terraced. The thickness of the retardation film 62 in thisparticular embodiment is greatest in the red subpixel and smallest inthe blue subpixel, thereby defining different retardation values for theretardation film 62 in the red, green, and blue subpixels which arematched to the particular wavelength of each corresponding subpixel.

Also shown in the fourth embodiment illustrated in FIG. 18, are a secondelectrode layer 64 deposited on the interior surface of the terracedretardation film 62, and a second orientation film 66 deposited on theinterior surface of the second electrode layer 64. The transparentelectrode film 64 and the transparent orientation film 66 are alsopreferably present in the other embodiments of this invention but havebeen omitted in the drawings for the purpose of simplicity. Theelectrode layer 64 combines with the matrix array 36 and electrodestherein to create a selectively activated voltage across each particularsubpixel. A first orientation film (not shown) is disposed on theinterior surface of the matrix array 36.

Each subpixel in certain embodiments of this invention preferably hasabout 7-9 possible gray level voltages. The same set of driving voltagesmay be used by each subpixel, or, alternatively, the set of gray leveldriving voltages for each subpixel may be chosen in accordance with thetransmission versus voltage curve of that subpixel. The presence of thepersonalized retardation films of certain embodiments of this inventionimproves the gray level performance of the LCDs by minimizing theinversion.

The second orientation film 66 acts in combination with the firstorientation film (not shown) deposited on the interior surface of matrix36 in that they preferably orient the liquid crystal molecules of the LClayer 38 at angles perpendicular to one another thereby creating about a90° twisted nematic liquid crystal cell. Like electrode layer 64, theorientation layer 66 and its corresponding first orientation layer (notshown) are present in certain other embodiments of this invention butare not illustrated in the drawings for purposes of simplicity.

The embodiment of FIG. 18 may, of course, be either a NW or NB type celldepending on the orientation of the transmission axes of the polarizers.

FIG. 19 illustrates a fifth embodiment of this invention. FIG. 19 is across sectional view illustrating a twisted nematic liquid crystaldisplay wherein the personalized subpixel retardation films 67, 68, and70 are made of different materials and therefore have differentbirefringent and retardation values. The retardation films and theirrespective birefringent values are chosen according to the wavelength oftheir corresponding subpixels. Therefore, this embodiment does notrequire the different retardation films to necessarily have differentthicknesses although they still may, depending upon the materials chosenand requisite birefringent values of the retardation films. Theretardation films 67, 68, and 70 of the blue, green, and red subpixelsrespectively, are located on the interior surface of the color filters42, 44, and 46, and on the exterior surface of the electrode layer 64and the orientation film 66. Like the other embodiments of thisinvention, this embodiment also may be provided with an additionalretardation film located, for example, between the second substrate 40and the second polarizer 56. Furthermore, it will be understood by thoseskilled in the art that the concept of this fifth embodiment whereineach retardation film is made of a different material having a differentbirefringent value may be applied to all other embodiments of thisinvention.

FIG. 20 illustrates a sixth embodiment of this invention. FIG. 20 is across sectional view of an embodiment of a twisted nematic LCD of thisinvention wherein the color filters 42, 44, and 46 of each subpixel ofthe pixel 31 are located on the second substrate 40, and wherein theretardation films 50, 52, and 54 of this embodiment are deposited on theinterior surface of the first substrate 34 on either side of the matrix36. Most preferably, the retardation films 50, 52, and 54 which arepersonalized according to the color of their respective subpixels, aredisposed on the interior side of the matrix layer 36. The retardationfilms of this sixth embodiment correct the horizontal components oflight before they enter the liquid crystal layer 38. It makes nodifference whether the retardation films are located on the firstsubstrate 34 or the second substrate 40.

FIG. 21 illustrates a seventh embodiment of this invention. FIG. 21 is across sectional view illustrating an embodiment of this inventionwherein the retardation films 50, 52, and 54 of the twisted nematic LCDare disposed between their corresponding color filters 42, 44, and 46,and the second substrate 40. Again, the net result is the same whetheror not the personalized retardation films are disposed below or abovetheir corresponding color filters in each subpixel.

A distinct advantage of this invention is that the retardation films maybe deposited directly on the color filters before assembly of the liquidcrystal pixel thereby allowing a manufacturer to prefabricate thecombination of the filter and the retardation film. For example, theretardation films 50, 52, and 54 may be spin coated onto color filters42, 44, and 46 to a desired thickness and retardation value before thecombination of the filter and retardation film is deposited or adheredto the corresponding substrate of the pixel. The soluble polyimides ofU.S. Pat. No. 5,071,997 which have negative birefringent values aresuitable for this purpose. This eliminates the need to deposit theretardation layers during manufacturing of the liquid crystal displaypixel 31.

FIG. 22 is a cross sectional view of an eighth embodiment of thisinvention illustrating a nematic liquid crystal display wherein thecolor filters and retarders are combined into integrally formed elements85-87. A single integrally formed polymer based element (85-87)functions both as a retarder and a color filter. This is accomplished byimmersing conventional color filter dyes into a polyimide material whichfunctions as a retarder. Soluble polyimides which may be used for thispurpose are disclosed in U.S. Pat. No. 5,071,997, which was previouslyincorporated herein by reference. It is known that the organic solventsoluble polyimides of the '997 patent may be used as retardation filmswhich have negative birefringent values. Conventional color dyes may bedissolved or immersed in these soluble polyimides thereby creating asingle integrally formed polyimide based element 85-87 which functionsas both a retarder and a color filter in a LCD. Color dyes which may beimmersed or added to these polyimides are disclosed in U.S. Pat. No.5,229,039, the disclosure of which is hereby incorporated herein byreference.

The retarder element of the retarders/filters 85-87 may or may not bepersonalized according to the wavelength of each subpixel in accordancewith the teachings of the other embodiments of this invention.Furthermore, an integrally formed polymer-based element which functionsas both a color filter and a retarder may be used in place of theseparate filters and personalized retarders of certain previouslydiscussed embodiments of this invention.

The polymer within which the color filter dye is immersed or dissolvedis preferably a polyimide. The polyimide is preferably either aco-polyimide or a homopolyimide. The homopolyimide is preferablyselected from the group consisting of: (i) a pyromellitic dianhydride(PMDA) and 2, 2′-bis (trifluoromethyl) benzidine (BTMB); (ii) 3, 3′, 4,4′-benzophenone tetracarboxylic dianhydride (BTDA) and 2, 2′ bis(trifluoromethyl) benzidine (BTMB); (iii) 4, 4′-oxydiphthalic anhydride(ODPA) and 2, 2′ bis (trifluoromethyl) benzidine (BTMB); (iv) 3, 3′, 4,4′-diphenylsu tetracarboxylic dianhydride (DSDA) and 2, 2′ bis(trifluoromethyl) benzidine (BTMB); (v) 3, 3′, 4, 4′-biphenyltetracarboxylic dianhydride (BPDA) and 2, 2′ bis (trifluoromethyl)benzidine (BTMB); and (vi) 2, 2′-bis (dicarbonylphenyl)hexafluoropropane dianhydride (6FDA) and 2, 2′ bis (tri-fluoromethyl)benzidine (BTMB).

The co-polyimide is preferably based on a material selected from thegroup consisting of: (i) 3, 3′, 4, 4′-benzophenone tetracarboxylic aciddianhydride (BTDA), 2, 2′-bis (trifluoromethyl) benzidine (BTMB) and 4,4′-diaminodiphenyl ether (DDE); (ii) 3, 3′, 4, 4′ biphenyltetracarboxylic dianhydride (BPDA), pyromellitic dianhydride (PMDA) and2, 2′-bis (trifluoromethyl) benzidine (BTMB); and (iii) 3, 3+, 4,4′-biphenyl tetracarboxylic dianhydride (BPDA), 2, 2′-bis(trifluoromethyl) benzidine (BTMB) and paraphenylene diamine (PPDA).

Furthermore, the polyimide is preferably organic solvent (e.g. m-cresol)soluble so as to simplify the process of immersing the color filter dyetherein. Such a material may be obtained from the University of Akron.

The birefringent and retardation values of the retarder aspect of thepolymer based elements 85-87 can be adjusted by the selection ofmaterials and by increasing or decreasing the thickness thereof.Furthermore, each of the different polymer materials discussed above hasa different birefringent value.

The color filter aspect of the polymer based elements 85-87 can beadjusted via the amount or type of dye immersed therein in accordancewith conventional methods.

FIG. 23 is a partial cut-away view of an LCD including a plurality ofpixels 310 of the different embodiments of this invention. The pluralityof pixels 310 are sandwiched between opposing polarizers 312 and 314,and between opposing transparent substrates 316 and 318. A liquidcrystal layer 320 is also disposed between the substrates and coversTFTs 322 which are used in the driving scheme of the pixels andsubpixels of the different embodiments of this invention. Row and columnlines 324 and 326 connect the TFTs 322. ITO electrodes 330 connect theTFTs to their respective pixels.

FIG. 24 is a graphic illustration of the angular relationship betweenthe “horizontal” (X) and “vertical” (Y) viewing angles discussed herein,and the conventional LCD viewing angles φ and Θ.

Once given the above disclosure, many other features, modifications andimprovements will become apparent to the skilled artisan. Such otherfeatures, modifications, and improvements are, therefore, considered tobe a part of this invention, the scope of which is to be determined bythe following claims:

We claim:
 1. A compensator for a liquid crystal display comprising: (a)a first deposited thin-film compensator layer having a first surface;(b) a second thin-film compensator layer deposited onto said firstsurface of said first compensator layer, wherein each of said first andsaid second deposited thin-film compensator layers are selected from thegroup consisting of: (i) a positively birefringent A-plate compensatorlayer, and (ii) a negatively birefringent C-plate compensator layer. 2.A liquid crystal display comprising: (a) a polarizer layer; (b) ananalyzer layer; (c) a liquid crystal cell having a first transparentsubstrate and a second transparent substrate forming respective walls ofsaid liquid crystal cell, said liquid crystal cell disposed between saidpolarizer layer and said analyzer layer; and (d) a compensator layer inaccordance with claim 1 disposed between said polarizer layer and saidanalyzer layer.
 3. A compensator element for a liquid crystal displaycomprising: (a) an optically transparent substrate; and (b) acompensator in accordance with claim 1, operatively coupled to saidoptically transparent substrate.
 4. A compensator of claim 3, whereinsaid optically transparent substrate is one surface of a liquid crystalcell.
 5. The compensator of claim 1, wherein another thin-film layer ofmaterial is deposited between said first deposited thin-film compensatorlayer and said second deposited thin-film layer.
 6. The compensator ofclaim 5, wherein said another thin-film layer of material is a depositedthin-film compensator layer.
 7. A compensator for a liquid crystaldisplay comprising: (a) a first deposited thin-film compensator singlelayer having a first surface; (b) a second thin-film compensator singlelayer deposited onto said first surface of said first compensator layer,wherein each of said first and said second deposited thin-filmcompensator layers are selected from the group consisting of: (i) apositively birefringent A-plate compensator layer, and (ii) a negativelybirefringent C-plate compensator layer.